The most fundamental, but often forgotten step towards a successfull microscopy session is the proper alignment of your microscope. In order to achive optimum system performance and acquire consistent, interpretable image data the illumination of the specimen must be adjusted to the most optimal, even level.
How to set up a microscope for köhler illumination should be an ingrained knowledge for every microscope user and should always be the first step of action once you switched on your microscope.
"How to set Köhler illumination" (by Humberto Ibarra Avila)
In this pdf you will find a step by step guide to properly set up your microscope for Köhler illumination and thus achieve optimal imaging results:
In this 12min video, Ron Vale talks you through a step-by-step procedure of aligning the lamp and condenser to achieve Koehler illumination.
August Köhler, who first introduced the Köhler illumination procedure, was part of the Carl Zeiss corporation. Here you can read how Zeiss Microscopy explains Köhler illumination.
This website explains Köhler illumination with the help of a nice, interactive Java tutorial.